1. Field of the Invention
The present invention relates generally to digital image compression systems, and more particularly to a system and method of compressing a document containing multiple types of images with compression schemes optimized for the characteristics of separate regions.
2. Description of Background Art
The archiving of documents has long been an important task in many business enterprises and other organizations. Recently, the archiving of digital images of documents has become possible, with affordable document scanners and fast computers. Retaining the full uncompressed digital image of documents, however, leads to difficulties with storage and efficient transmittal across networks. For this reason, most digital document archives employ some sort of compression scheme to decrease the amount of memory required for a single digital document image.
A number of techniques are used to compress digital images, and they generally fit into two broad categories: lossy and lossless. Lossy techniques allow some image data to be lost in the process of compression, while with lossless techniques there is no loss of image data. With lossless techniques the decompressed image is identical to the original. Compression techniques tend to be optimized for specific types of images. One of the image types is "grayscale," meaning that an image is composed of many different shades of gray. A typical grayscale image is a "black and white" photograph of a real world scene, in which the shading of objects is characterized by subtle changes from white to light gray to dark gray to black. Grayscale images are a special case of "continuous tone" images. Continuous tone images contain many colors, but only those colors which are on a gradient between two distinct colors. A continuous tone image could, for example, be composed entirely of color gradations ranging from green to yellow. In the case of grayscale images, the two distinct colors are black and white, with the colors in the image being some combination of black and white. Most instances of non-grayscale continuous tone images are found in images created by graphic artists, such as in some magazine advertisements. Another type of image is referred to as "bitonal." A bitonial image is composed entirely of two distinct colors. The most commonly seen example of bitonal images is probably black text on white paper. Bitonal images are characterized by sharp contrasts, for example the edges of the letters on a printed page. Yet another type of image is the "full color" image. Full color images contain a large number of very different color values, as in a color photograph of a real world scene. A full color image is similar to the continuous tone images, in that it is characterized by smooth variations in color, but the colors are not limited to those on a gradient between two distinct colors. In a typical magazine or newspaper page multiple types of images are present: the photographs and artwork tend to be continuous tone or full color, and the text tends to be bitonal. An image which is composed of more than one type of image is referred to as a "hybrid image."
For the compression of continuous tone and full color images, maximum compression with acceptable results can generally be achieved with lossy compression schemes, such as the Joint Photographic Experts Group (JPEG) standard. This standard is described in J. D. Murray and W. VanRyper, Encyclopedia of Graphics File Formats, Second Edition, O'Reilly & Associates, Inc., Bonn:1996 and ISO/IEC 10918-1:1994, Information Technology--Digital Compression and Coding of Continuous-tone Still Images: Requirements and Guidelines. The loss of image information in lossy schemes such as JPEG does result in the loss of some image detail. After decompression, however, the result of a JPEG compression performed on most grayscale and full color images is an image which appears to the human eye to be nearly identical to the original. When JPEG (or other lossy compression schemes) are applied to bitonal images, such as a page of text, the results are poor. The sharpness of the characters suffers and the text appears fuzzy. Other loosy compression schemes include fractal compression, described in Yuval Fisher, ed., Fractal Image Compression: Theory and Application to Digital Images, Springer Verlag, New York, 1995; and wavelet compression, described in C. K. Chui, An Introduction to Wavelets, Academic Press, London, 1992.
Bitonal images are generally compressed with a lossless scheme, in order to preserve the sharpness of the image. Many lossless schemes are specifically optimized for bitonal images and do not handle grayscale and full color images well. Examples of such schemes are the Joint Bilevel Imaging Group (JBIG) standard, described in International Telecommunication Union Recommendation T.82; and the Group 3 and Group 4 standards (G3 and G4) of the 1-D Consultative Committee of the International Telephone and Telegraph (CCITT). These schemes are described in Encyclopedia of Graphics File Formats and R. C. Gonzalez & R. E. Woods, Digital Image Processing, Addison-Wesley, Reading, Mass.: 1992. While these standards result in significant compression of bitonal images without loss of detail, they perform poorly on grayscale and full color images, sometimes even resulting in negative compression (i.e., larger files).
When scanning and compressing images containing more than one type of image, as in a typical magazine or newspaper page, a tradeoff must be made. If lossy techniques are utilized, the image quality of the text portion of the page suffers, and if lossless techniques are utilized, less compression is achieved. What is needed is a way to maximize compression of hybrid images without loss of detail in the bitonal portions.